This invention relates to a method of producing a polar anisotropic rare earth magnet, and more particularly to a method of producing a cylindrical or annular permanent magnet which is magnetized in radial directions and has polar anisotropy by compacting and sintering a magnetic powder essentially composed of Nd, Fe and B. The rare earth permanent magnet is suitable for use in motors for electric and electronic devices.
Several methods have already been proposed for producing sintered, cylindrical or annular permanent magnets with radial orientation or polar anisotropic orientation. For example, in compacting a magnetic powder in a cylindrical mold with a hydraulic press a known method to realize polar anisotropic orientation is arranging several poles of electromagnets around the side wall of the mold, and a known method to realize radial orientation is arranging two electromagnets axially of the mold to interpose the mold therebetween with the same poles of the respective magnets directed toward the mold to utilize repulsion between the magnetic fields. However, these methods have disadvantages such as the need of using large-sized electromagnets, difficulty of varying the number of poles and insufficiency of orientation because of difficulty in producing magnetic fields of desirably high intensity.
In the case of producing a plastic magnet by injection molding the magnetic powder in the melted resin can be oriented to a sufficiently high degree by applying magnetic fields of relatively low intensity. However, because of containing a binding resin which is a nonmagnetic material plastic anisotropic magnets are inferior to sintered anisotropic magnets in magnetic characteristics.
It is well known that a magnetic field of very high intensity and very short duration, viz. a pulse of magnetic field, can be produced by instantaneously supplying a large current to a coil from a capacitor. For producing a polar anisotropic magnet, JP-A 59-216453 proposes to repeatedly apply a pulse of magnetic field to the magnetic powder under compression with a static press means such as a hydraulic press to thereby induce polar anisotropic orientation. However, by this method the orientation becomes disordered because the duration of each pulse of magnetic field is far shorter than the duration of pressing and also because the magnetic field can be produced only intermittently during the compressing process.
In view of the above problems, we have already proposed to use a pulse of magnetic field and a pressure pulse in combination for shaping a magnetic powder into a cylindrical green body with polar anisotropic orientation. (JP-A 58-157901, JP-A 61-243102 and JP-A 61-241905.) However, in the case of producing a polar anisotropic rare earth magnet essentially composed of Nd, Fe and B, there is another problem which is attributed to anisotropy of shrinkage of the sintered magnet body. That is, the sintered body exhibits a greater amount of shrinkage in the direction of magnetization than in the direction perpendicular thereto. Therefore, when an annular green body is sintered the sintered body has dents and projections on its side surface and hence, with exaggeration, has a petaloid shape in plan view. With such deviation from an annular shape the magnet can hardly be used in rotating machines.